A Treatise On The Principles And Practice Of Harbour Engineering

I82 HARBOÜR ENGINEERING. sunk in position, they afforded a margin of 6 feet 6 inches above low-tide level. The material of the caissons was Bessemer steel in plates | inch thick, strengthened internally by longitudinal and transverse bulkheads of lattice- work so as to form six equal compartments. In each of these compartments were subsequently placed two concrete blocks of about 40 cubic yards volume. The rubble work bed on which the caissons were to rest was levelled ready to receive them through the agency of a diving-bell. The time occupied in this operation varied from one to two days. The caisson, having been ballasted, as already stated, was then towed into position during the last two hours of an ebb-tide, so as to allow as much time as possible for its]adjust- ment and settlement. Sinking was effected by pumping water into the Plan Fig. 156.—Bilbao Breakwater, showing Titan and Caisson. interior of the caisson through a centrifugal pump suspended from a Titan crane and worked by an electric motor. If irregular settlement took place, the action was reversed and the caisson refloated until its misplacement had been rectified. Once truly placed, eight or ten 40-cubic-feet blocks were deposited in the compartments of the caisson by the same crane. This generally absorbed the whole of the time available on the tide. The following tide, after pumping out the caisson, the remaining blocks were deposited and liquid concrete was run in between them and the others with a solid layer, 20 inches thick, covering the whole. During the third period of low water the superstructure was taken in hånd. It consists of two facings of concrete blocks, approximately 13 feet by